I run a computer shop and you simply cannot compete with pricing from the big stores (pre-builts - Walmart, Best Buy, online parts - Newegg, Amazon, etc). Even through my vendors, I usually don't save more than a few dollars on most hardware compared to Newegg or Amazon. However, when I order from them, I usually need to place an order of $800 - $1,000 in order to get free shipping. So I can't just order a few parts from one of my vendors as purchasing that same few items from Amazon or Newegg will likely be cheaper once shipping is factored in.
Remember, all the pre-builts are made in China. Labor is cheap and parts are bought by the truckload so they are heavily discounted as well. You will never be able to offer the same hardware at the same price as them, especially on the low end ($500 or less). You have to offer something they can't.
People always come in and ask why should they buy a computer from me vs going to Walmart or Best Buy. I tell them I support what I sell. If they ever need help with something or have an issue come up, I will be there to help them. Either by going out to their house, remoting into their system or walking them through something on the phone. I will have someone there to help them. Our "low end" system is built with the same or similar performing specs they would find at a big store but with quality components and a build quality that exceeds anything they can purchase without going to a boutique builder (Alienware *sort of*, Falcon, Origin, Ibuypower *shudders - they should rename to ibuycrap* etc.) and spending a good bit more money.
The real money comes from support. People always need work done on computers. Whether it's removing virus/spyware/crapware/malware to optimizing a system, transferring data or just simply reinstalling an OS. People always need that kind of stuff done. More than what you would think. Most people that buy a computer from us also have us service it. Repeat business is usually good business.
Just as an example on making money between building systems or working on them. We make less than $100 in profit on most builds and have on average about 6 new builds a month. If we stopped building new systems and it had no other affect on the business, it would be less than 6% of our total income. Now, I don't run a huge business and we aren't in a heavily populated area (population for the entire county is less than 22,000) but we would do well and stay busy just servicing personal/business computers and setting up and maintaining networks.
The service side is where the money is at.